1. Field of the Invention
The present invention relates to a plugged honeycomb structure, and more particularly, it relates to a plugged honeycomb structure which can collect ash and particulate matter (PM) in large amounts and in which an increase of a pressure loss is suppressed.
2. Description of Related Art
In gases discharged from an internal combustion engine such as a diesel engine, various burning devices, and the like, a large amount of particulate matter (PM) mainly composed of soot is included. When this PM is discharged to the atmosphere as it is, an environmental pollution is caused. Therefore, in an exhaust system of the exhaust gas, a filter to collect the PM (e.g., a diesel particulate filter (DPF)) is disposed.
As the filter, there has been used a plugged honeycomb structure having porous partition walls with which a plurality of cells are formed to become through channels of fluids (an exhaust gas and a purified gas), and plugged portions to plug open frontal areas of the cells. The plugged portions are arranged in the open frontal areas of the predetermined cells (inflow cells) in an end surface on an outflow side of the fluid (the purified gas), and the open frontal areas of the remaining cells (outflow cells) in an end surface on an inflow side of the fluid (the exhaust gas).
In the above plugged honeycomb structure, when the exhaust gas is allowed to flow into the inflow cells, the PM in the exhaust gas is collected by the partition walls during passage of the exhaust gas through the partition walls, and “the purified gas from which the PM has been removed” flows out of the outflow cells.
Moreover, as the plugged honeycomb structure in recent years, there has been reported a plugged honeycomb structure constituted of two types of cells including large capacity cells having a relatively large area in a cross section perpendicular to a longitudinal direction, and small capacity cells having a relatively small area in the above cross section (see Patent Document 1). In this plugged honeycomb structure, the number of the large capacity cells is about the same as that of the small capacity cells. In this plugged honeycomb structure, a pressure loss during the collection of the ash, PM and the like is low, and the ash and PM can be collected in large amounts, as compared with a plugged honeycomb structure having one type of cell shape (e.g., an only quadrangular shape) in a cross section of the structure which is orthogonal to a cell extending direction. Hereinafter, “the plugged honeycomb structure having the one type of cell shape (e.g., the only quadrangular shape) in the cross section of the structure which is orthogonal to the cell extending direction” will be referred to as “a conventional plugged honeycomb structure” sometimes.    [Patent Document 1] WO 2004/024294